MOSCOW — Russians voting in parliamentary elections apparently turned against the ruling United Russia party in large numbers Sunday, exit polls and early results suggested, to the great benefit of the Communist Party.

In what only months ago would have been a nearly unimaginable scenario, the party dominated by Vladimir Putin was predicted to get less than 50 percent of the vote, while polling organizations put the Communists at about 20 percent, nearly double their count in the last election.

Russia’s only independent election monitor called the vote the most flawed to date, an assessment echoed by opposition leaders, implying that United Russia’s actual haul would have been even less had the election been free and fair.

Not long ago, anything under the 64.3 percent that United Russia won in 2007 would have been seen as unacceptable failure for the party and Putin, who has relied on its control of government and bureaucrats across the country to deliver ever more votes and entrench his authority.

But now its aura of invincibility is badly dented, and opponents may begin to sense an opportunity. If United Russia falls short of 50 percent of the seats in the lower house of parliament, it will turn to the nationalist Liberal Democrats, or even the Communists, for support. Those parties have been pliable up to now — Vladimir Zhirinovsky’s Liberal Democrats never vote against the government — but could start testing the limits of their power, given a chance.

Disenchanted electorate

A chastening by voters probably will require Putin to pay closer attention to domestic politics and the time-consuming tasks of improving the economy and trying to tamp down corruption, perhaps making abrasive international encounters less appealing. Although Putin and President Dmitry Medvedev employed anti-U.S. sentiment during the last days of the campaign, with sharp rhetoric about NATO’s plans for a missile defense system in Europe and dark allusions to Western meddling in the elections, it produced little resonance among voters.

Voters began to grow disenchanted this year with a stagnating economy and unkept promises, even as they appeared too apathetic to do anything about it. But in mid-September, the Kremlin began to make missteps, ineptly removing billionaire Mikhail Prokhorov as head of the Right Cause party, then announcing that Putin would switch places with Medvedev and run for president in March. Russians began to think that Putin, who had been president from 2000 to 2008, might be in power too long.

Throughout the summer, United Russia strategists had been telling party activists that they had to draw 60 percent of the vote, giving the bloc enough seats to be able to rewrite the constitution at will. As late as Oct. 3, the Vedomosti newspaper quoted Kremlin sources as saying that the party had set a goal of 65 percent. But confidence was waning; there was no more talk along those lines after that. Some analysts began to think that Medvedev, who headed the party list, was being set up to take the blame if the results fell short.

The opposition blogger Alexei Navalny said in October that the election results could force United Russia to negotiate and compromise with others. “If they get less than 60 percent, they won’t be able to behave as if there were no other political parties around,” he said, according to the online news organization Gazeta.ru.

United Russia’s weaker showing has a bearing, as well, on the March 4 presidential election. Putin has always been more popular than the party, but if he is unable to do better than early Sunday results indicate and falls short of 50 percent, that will force a runoff vote with the No. 2 finisher. It would almost certainly stir comparisons to the 2004 Ukrainian presidential election, which led to the Orange Revolution. For seven years, Putin has been determined to avoid anything similar in Russia.

The United Russia party put on an impassive face as its less-than-desired results were reported Sunday. “Resting on this result, we will be able to secure a stable development of our state,” Putin said at a briefing for Russian reporters.

In the evening, exit polls published by government-friendly pollsters predicted United Russia would win 48.5 percent; the Communist Party, 19.8 percent; A Just Russia, 12.8 percent; and the Liberal Democratic Party, 11.42 percent. Early Monday, the Central Election Commission reported 49.9 percent for United Russia, with about 70 percent of the votes counted.

Irregularities cited

Reports of irregularities also flowed in. Yelena Panfilova, who runs the Russian office of Transparency International, said she saw a man ahead of her in line stuffing absentee ballots into a ballot box, the Interfax news agency reported.

There was extensive evidence of official panic as the election approached. Over the past week, an intense and coordinated attack was launched against Golos, the nation’s only independent election monitor. On Friday, a television station with ties to the Kremlin attempted to discredit Golos because it receives funds from the United States and Europe. The same day, a court found it guilty of violating election laws.

The campaign, however, seemed to introduce Golos to those who had never heard of it and anger those who had. “They are blamed for receiving money from abroad,” Dmitry Muratov, editor of Novaya Gazeta, told Echo Moskvy radio. “Well, it is always better to receive foreign funds than steal money from the Russian national budget.”On Sunday in Moscow, protesters were arrested before they could assemble and reports surfaced of hospital patients being given filled-out ballots while doctors watched to ensure they cast them. Demonstrators were arrested in St. Petersburg.

Early Saturday, customs officials at the airport detained Lilia Shibanova, executive director of Golos, as she was returning from a trip and confiscated her laptop.

“These elections are the dirtiest ever,” she said Sunday. “It cannot go on like this.”

Early Sunday, the Golos Web site was shut down by hackers, along with several sites operated by independent information providers, including that of Echo Moskvy radio — its Web site went back up after the voting ended.

When the Golos Web site disappeared, so did its online map that registered complaints of election violations. Employees moved to an independent press center, where they took complaints by phone.

By 8 p.m. Sunday, 2,058 calls had come in, with many complaints about the misuse of absentee ballots, monitors barred from polling places and sightings of young people being bused from one polling station to another to vote more than once.

The count was expected to be complete by Monday, but it appeared that a longtime opposition party, Yabloko, would once again fail to clear the 7 percent threshold for seats in the Duma. Still, opposition leaders were feeling victorious.

“The end of this regime will, in fact, start on December 5,” said Mikhail Kasyanov, a former prime minister and current opposition politician whose party was not allowed to take part in the vote.

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